MSE research group studies aging mechanisms of nuclear power plant cable insulation

Nuclear power plants are an important source of energy production in the United States and elsewhere, and their operational safety is important for the prevention of nuclear disasters. In a typical nuclear power plant, there are around 1,000 km of power, instrumentation and control cables whose integrity is necessary for safe operation of the reactor. How durable are cables under the harsh environment of a nuclear reactor? Do they age well? What is the quality of the cable materials that were constructed and implemented around forty years ago when many United States nuclear power plants were built?

A diverse and interdisciplinary team of researchers led by Dr. Nicola Bowler, Professor in the Department of Materials Science and Engineering at Iowa State University, has begun to investigate the way in which Cross-Linked Polyethylene (XLPE) cable insulation ages in nuclear power plants.

“This project was built on two other major pieces of work that we had in the area of polymer aging in wiring applications originally funded by NASA and The Boeing Company. This work is about aging concerns in polymers in relation to signal transmission and disasters that can happen if the cables do not work,” Bowler said.

Bowler and her group of students at Iowa State have been working diligently on this project since October 2014, and it is set to be complete by September 2017.

The research team includes: Shuaishuai Liu, a fourth-year Ph.D. student in materials science and engineering; Zhihui Shao, a first year Ph.D. student in electrical engineering; Chamila De Silva, a master’s student in materials science and engineering; Michael Byler, an undergraduate student in mechanical engineering and Mario Imperatore, a five-month visiting master’s student in electrical engineering from Bologna, Italy. In addition to the group of Iowa State students, there is a second investigator and lab technician at Pacific Northwest National Laboratory (PNNL). The group also collaborates with Dr. Thomas Chiou of the Center for Nondestructive Evaluation and Dr. Scott Beckman, a former faculty member in the Department of Materials Science and Engineering and currently an Associate Professor at Washington State University.

“Each of us does our own job on our own time, but we all have very good communication within the group. As a student with a materials science and engineering background, I start to think of the project from the materials science point of view, but then I discuss a question with the electrical engineering students. That encourages me to test the electrical properties of the material I was working on, and then we discuss the results together,” said Liu.

Bowler said, “At my level, I find working with a team with a variety of backgrounds important too. Having a co-investigator at PNNL helps because his specialty is polymer chemistry, and I am more of a physicist and a measurement scientist. The polymer chemistry is not my strength. The shared expertise is really what makes it work.”

Most nuclear power plants in the United States were built around forty years ago and the initial license issued to each plant qualified them for forty years of operation. Recently, there has been a rise in the number of nuclear power plants requesting license renewals, but the current renewal process, to receive an additional twenty-year operating license, is not cheap and takes around four years to complete. Operational safety is critical for nuclear power plants; therefore, a significant amount of research is being conducted on the safety inspection of the cables.

Bowler said, “There are many types of cables in a structure like a nuclear power plant, as you might expect. Some are power cables, but our research focuses more on the instrumentation and control cables. An incident like the Fukushima Daiichi nuclear disaster, that escalated because they lost power in the plant, reminds us of the importance of having cables function at the end of the licensing period even if an “incident” occurs on the last day. Our work is related to ensuring cables’ long-term performance – we make the polymer material components age rapidly and then study how they perform under those conditions.”

One emerging method for testing nuclear power plant cables involves measuring so-called “indenter modulus.” This testing method is only valid for some types of material, however.

“For example, the material that we are focusing on is XLPE. From our [and other] research, the indenter-modulus test is not valid for this type of material. We have found a more effective non-destructive method for this particular material,” said Liu.

The research team started testing XLPE cable materials after exposure to the conditions at the nuclear power plants that induce aging such as elevated temperature and ionizing radiation. Cable polymers degrade faster under these conditions. The group is studying aging mechanisms and asking questions such as: Why do the cables age in that condition? How are they aged? What factors play into cable aging? The last part of the project is to identify a way to inspect the integrity of the cable insulation through its outer protective jacket, to complement the indenter-modulus method that inspects the cable jacket alone.

Environmental aging experiments in which the environment in the nuclear power plant is simulated, to age the samples, were conducted at the Pacific Northwest National Laboratory. Strong stressors were applied to the cables in a relatively short time (up to 25 days) to achieve similar aging severity to forty-year-old cables in a typical nuclear power plant. After the samples were aged in a variety of different conditions, they were sent back to Iowa State, and the group studied the polymer aging mechanisms through several materials characterization experiments and measured properties of the cables that might be good nondestructive indicators of their aged condition.

After tests were completed and the material properties evaluated, the next step was to incorporate the two electrical engineers, Imperatore and Shao, to design different ways to inspect cable insulation.

“We have seen that the capacitance and dissipation factor are often well correlated to the aged state of the cable. These are the electrical properties of the polymer. We are used to thinking of metal as a material that conducts electricity, but polymers do the opposite. They are insulators. They often start to conduct electricity if they are becoming degraded. Properties that normally indicate a good insulator start to change as the material gets old, and we can test that through electrical testing methods,” said Bowler.

Imperatore determined a way to assess the electrical properties of the cable insulation polymers by unhooking the cable to access the conductors at its end and measuring dissipation factor and capacitance by applying a voltage across the conductors. If it is undesirable to move the cable during a test, then Shao’s method of testing works better. He has designed a sensor that is placed on the exterior surface of the cable. The method uses polarized electrodes whose capacitance can be measured and the measured capacitance then correlates to the aging severity of the cable. This approach gives more localized measurements of cable properties, and there is no need to access the central connector. The methods are complimentary to one another.

Bowler stated, “It is a work in progress to determine if this method works on other materials. We’ve mostly studied XLPE in this project. In other projects, we have looked at other polymers as well but not the fully exhaustive list of all the materials that are used in practice. There are other methods that are used in the nuclear power plants at the moment but ours offers potential improvements.”

With six months left of the current project, the future for the research is bright. Bowler has applied for a three-year renewal through the Department of Energy’s Nuclear Energy University Program. If that is funded, Byler would continue to work with the project in the future. De Silva hopes to be finished with her master’s degree by summer, and Liu expects to complete her Ph.D. in August 2017. Imperatore has returned to Bologna, Italy, to finish his thesis and give his final presentation. Shao’s next step is to fabricate and test the sensor that he has designed. Bowler will continue to partner with inspection companies to transfer the technology to a commercial test instrument so that the results of this project can be applied to advance cable testing in practice.

Bowler spoke highly of the quality of research that they have done. She mentioned that the team has answered many of the research questions that she wanted to cover, and now they are in the writing phase to present the results in the public domain. What is next for Bowler and her research group? Testing other materials.

“Another international group that does work in this area is located in Japan. A Japanese professor wrote a paper a few years ago on nuclear power plant cables and his paper was instrumental in encouraging me to apply for this funding. I hope to develop a more concrete relationship with the Japanese group as well as the group in Italy,” said Bowler.

As the students wrap up the testing and data analysis and Dr. Bowler awaits the announcement in relation to future funding, the group looks forward to presenting final results, building international and industrial partnerships and determining ways to test additional cable materials.

Shuaishuai Liu beamed about Dr. Bowler saying, “I think Dr. Bowler is a very good leader. I think she is very good at getting the direction of our group to be accurate and seeing the big picture. Her direction has led us to our success.”